Intestinal toxicity is a major side effect of cancer chemotherapy. The procedures to isolate and cuture rat small intestinal epithelial crypt cells, which are the target cells for drug toxicity, have recently been developed. The overall objectives of this study is to establish an in vitro method to quantify the in vivo intestinal toxicity of anticancer drugs. The phase I study is to examine the feasibility of using an in vitro toxicity assay to predict the intestinal toxicity of anticancer drugs using the rat as an animal model. The research plan is to evaluate and correlate the druginduced rat intestinal crypt cell toxicity in vitro and the intestinal disturbances in intact animals. Two antimetabolites, 5fluorouracil and methotrexate, are selected as the test drugs. Drug toxicity is determined by the concentration as well as the duration of exposure to the drug. In order to make valid comparison of the in vitro and in vivo toxicities, the in vitro and in vivo studies will be done under similar drug concentrationtime (AUC) profiles. The crypt cytotoxicity and the intestinal disturbances, observed under the same AUC of the drug, will be compared and a qualitative and quantitative correlation will be sought. The phase II study will examine the general applicability of the in vitro assay using at least three drugs from each of the major drug classes including antimetabolites, alkylators and DNA reacting agents, and compare the rat and human cells. Comparison of the cytotoxic responses of rat and human crypt cells is necessary to understand the applicability and limitation of the crypt cytotoxicity assay to predict for drug toxicity in patients.